1. Field of the Invention
The present invention relates in general to the poling of polymeric film materials that exhibit at least one of pyroelectric and piezoelectric properties, and in one aspect relates to the continuous poling of such film materials.
2. Description of the Related Art
Several polymeric film materials exhibit both pyroelectric and piezoelectric properties. However, some polymeric film materials may exhibit only one of such properties.
The pyroelectric and/or piezoelectric property of polymeric film materials is commonly referred to as being caused by an arrangement of the dipoles of the film. Normally the dipoles of pyroelectric and/or piezoelectric polymeric films are essentially arranged in random fashion. However, such randomly arranged dipoles can be rearranged into a poled orientation by heating the film above a particular temperature, known as the poling temperature, and concurrently applying an electric field across the film. Once the film is poled and cooled below the poling temperature, the electric field may be removed and the dipoles will remain substantially as oriented by the field to yield film with desirable pyroelectric and/or piezoelectric properties. However, if the poled film is again heated above its poling temperature for an extended period thereafter, the dipoles will return to substantially random orientation.
It is generally known that the degree of dipole orientation that results from such a poling process is a function of the temperature to which the film is heated, the magnitude of the applied electric field and the length of time such electric field is applied while the film is heated above the poling temperature. For example, substantial poling occurs in a film of polyvinylidene fluoride when it is heated to a temperature of 90.degree. C and an electric field of 400 K volts/cm. is applied across the film for 10 minutes. In general terms, increasing the temperature, the electric field magnitude, and/or the poling time will progressively increase the degree of poling achieved up to a maximum.
The art describes poling of pyroelectric and/or piezoelectric films in non-continuous modes. For example, U.S. Pat. No. 3,878,274 indicates that polyvinylidene fluoride film can be poled by placing electric fields of 50 K volts/cm. to 2000 K volts/cm. across the film at temperatures between 40.degree. C and 150.degree. C. However, such patent contains no detailed teaching of the particular magnitude of electric field that is used at a particular temperature. Accordingly, since the electric field strength that films can stand without breakdown and arcing therethrough is directly related to the temperature of the sheet of film being poled, it is not apparent from such patent how to create an improved apparatus and/or process for poling. The examples given in the U.S. Pat. No. 3,875,274 pole at 700 K volts/cm. at 90.degree. C for 30 minutes, 400 K volts/cm. at 90.degree. C for 30 minutes, and 200 K volts/cm. at 150.degree. C for apparently 30 minutes also.
An Applied Physics Letters article entitled "Effects of Poling Conditions on Responsivity and Uniformity of Polarization of PVF.sub.2 Pyroelectric Detectors" (Vol. 24; No. 10; May 15, 1974) indicates that the uniformity of polarization through the thickness of polyvinylidene fluoride (PVF.sub.2) film material is related to the responsivity of the poled material. The article also points out that very uniformly poled material can be obtained by poling at 2100 K volts/cm. at 70.degree. C for 5 minutes, but does not indicate the rate of material breakdown experienced. Furthermore, the article states that increased temperature and voltage tend to make polarization more uniform; and that increased poling time likewise tends to make polarization more uniform.
Other art has indicated poling results for short periods of time in the seconds range and compared this data to data for longer periods of time in the several minutes range using the same poling temperature and electric field magnitude. A Journal of Applied Physics article entitled "Pyroelectricity in Polyvinylidene Fluoride" (Vol. 44; No. 5; May, 1973) indicates in FIG. 10 that the magnitude of polarization achieved by the use of short poling periods is only a fraction of the magnitude achieved for long poling periods. A Journal of Polymer Science article entitled "Persistent Polarization in Polyvinylidene Fluoride" (Vol. 13; 1975; pp. 1033-1047) indicates similar findings.